As the development of biometric devices for identity verification, and in particular of fingerprint sensing devices, has lead to devices which are made smaller, cheaper and more energy efficient, the possible applications for such devices are increasing.
In particular fingerprint sensing has been adopted more and more in, for example, consumer electronic devices, due to small form factor, relatively beneficial cost/performance factor and high user acceptance.
Capacitive fingerprint sensing devices, built based on CMOS technology for providing the fingerprint sensing elements and auxiliary logic circuitry, are increasingly popular as such sensing devices can be made both small and energy efficient while being able to identify a fingerprint with high accuracy. Thereby, capacitive fingerprint sensors are advantageously used for consumer electronics, such as portable computers, tablet computers and mobile phones, e.g. smartphones.
A fingerprint sensing chip typically comprises an array of capacitive sensing elements providing a measure indicative of the capacitance between several sensing structures and a finger placed on the surface of the fingerprint sensor. The sensing chip may further comprise logic circuitry for handling addressing of the array of sensing elements.
A typical fingerprint sensor is protected so that the finger does not come in physical contact with the sensing elements. In particular, it may be desirable to arrange a glass plate on top of the sensor for protecting the sensor, or to arrange the sensor behind a display glass. By arranging elements between the sensing surface and the sensing elements, the distance between the sensing surface and the sensing elements increases which reduces the capacitive coupling between a finger placed a sensing surface of the device and the capacitive sensing elements. This in turn leads to an image blurring effect. As a function of an increased distance between a finger and any given pixel, each pixel is starting to receive signals from areas that are not immediately located vertically on top of said pixel resulting in image blurring negatively impacting the capabilities of the sensors to resolve fine features in a fingerprint.
In view of the above, it is desirable to improve the capacitive coupling between a finger placed on the sensing surface and the sensing elements.
US2013/0201153 discloses a fingerprint sensing device where electrically conductive strands are arranged between the sensing surface and the sensing elements of a fingerprint sensing device. An insulating material is arranged between conductive strands. However, a direct electrical contact between the finger and the pixel may cause problems related to electrostatic discharge (ESD). Moreover, the metallic portions of the surface may oxidize, resulting in undesirable aesthetic effects.